This invention relates to heating ventilating and air conditioning (HVAC) systems and more particularly to controls for controlling such HVAC systems.
Electronic thermostats are generally known in the art. Typical construction of such an electronic thermostat 10 is shown in prior art FIG. 1. A processor 15, usually a microprocessor is connected to a memory 20 and sensor 25 a display 30 and an input output device 35. The processor controls overall operation of the thermostat and produces a control signal which is passed through input output (IO) device 35 to the HVAC plant for controlling the operation of the plant. The memory 20, stores instructions on which the processor operates. Sensor 25 generates an temperature signal representative of the temperature of the air in the vicinity of the sensor. Display 30 displays information to an operator of the thermostat. This information may include the current setpoint, the actual temperature sensed by the sensor 25, the operating status of the HVAC plant and the like. IO device 35 receives signals intended for the HVAC plant from the processor and converts those signals into control signals for the HVAC plant. IO device 35 also receives signals from the HVAC plant and converts those signals into signals which processor 15 can interpret.
Some U.S. Pat. Nos. which depict such a thermostat are 4,332,352 issued Jun. 1, 1982 (Jaeger), 4,387,763 issued Jun. 14, 1983 (Benton), 4,702,305 issued Oct. 27, 1987 (Beckey, et al '305), 4,702,413 issued Oct. 27, 1987 (Beckey, et al '413), 4,828,016 issued May 9, 1989 (Brown, et al) and 4,911,358 issued Mar. 27, 1990 (Mehta).
Typically, for a single stage HVAC plant, a control scheme such as that shown in prior art FIG. 2 was used. Thermostat 15A subtracted a temperature sensor signal T.sub.sen from a setpoint, T.sub.set using summer 100 to create an error signal, err. The error signal was then used in a proportional integral filter 105, 110. Filter 105 produced a proportional error signal, errp. Filter 110 produced an integral error signal erri. errp and erri were summed at adder 115 to produce a proportional integral error signal, errpi.
Anticipator 130 was used to create an anticipation signal, ant, based on the on/off status of switch 125. The anticipator is an optional feature and is normally used in the thermostat art. The anticipator is any type of signal source compatible with the adder 120 and it is energized upon operation of the HVAC plant and supplies a lag signal as negative feedback to anticipate the operation of the system.
The ant signal is subtracted from the errpi signal to create a switch error signal, errsw. Switch 125 then compares errsw with a predetermined switch setpoint, sw and turns on if a predetermined relationship, such as greater than, exists between errsw and sw. If the switch turns on, an on signal is sent to the HVAC plant to initiate HVAC plant operation.
Now referring to FIG. 3, the advent of multistage HVAC plants, 150b, 151, 152 required multiple switching capabilities. This led to inclusion of switches 126 and 127 in addition to switch 125b over the system described in FIG. 2. Each switch 125b, 126 and 127 did its own comparison of errsw to its own switch point sw1, sw2, sw3, to determine when the switch should turn on its respective stage. However, all three switches received the same errsw signal.
Mechanical thermostats, such as Honeywell's T87 The Round.TM. thermostat, worked in a slightly different way when used in a single stage configuration. Prior art FIG. 4 shows the control scheme for such a mechanical thermostat. Summer 405 subtracts setpoint temperature T.sub.set from the sensor temperature T.sub.sen to create a droop signal. The droop signal is then fed into summer 415 from which the anticipator signal, ant, is subtracted to produce an error switch signal, errsw. Note the lack of the proportional and integral filters in the mechanical thermostat. For multistage applications, mechanical thermostats used a control scheme which in essence provided one of the devices shown in FIG. 4 for each stage of the HVAC plant.
Referring now to FIG. 5, there shown graphically is a comparison between the control point for three stages in an electronic thermostat vs. the load of the three stages being controlled by the prior art multistage electronic thermostat of FIG. 2. Line 500 shows the setpoint. Line 501 shows the effect on the load of each stage of the multistage HVAC plant using full integration of the err signal. Line 502 shows the effect on the load of the three stages using only a partial integration of the err signal. Line 503 shows the effect of loading on the three stages in the multi plant HVAC system when no integration of the err signal is used.
Referring now to FIG. 6, there shown is a load vs. control point graph of a mechanical thermostat. Note that there may be substantial droop between the setpoint Line 600, and the load vs. control point Line 601 and 602 in the mechanical thermostat. However, the mechanical thermostat must provide the ability to maintain a certain separation between the point at which stage one is at 100 percent load and the point at which stage two initiates. This is called the "interstaging" and is represented by the difference identified at 605. This kind of interstaging has been only available to date in mechanical thermostats.
There are times when some combination of the benefits provided by a mechanical thermostat and the benefits provided by an electronic thermostat in terms of ability to control load and droop are desired. Accordingly, it is an object of the p resent invention to provide a thermostat which is configurable to provide elements as desired of both mechanical and electronic thermostatic control.